The invention relates to a digital/analog converter with programmable gain for converting a digital input signal into an analog output signal.
FIG. 1 shows a conventional current-controlled digital/analog converter DAC 1 according to the prior art. The digital/analog converter 1 shown in FIG. 1 contains a reference current source 2 which outputs a reference current to a current mirror array S 3. The current mirror array S 3 contains current mirror circuits which mirror the generated reference current with an associated mirror ratio to form mirror currents and output them via an associated controllable switch to a summation current node. In this case, the controllable switches are driven by n lines connected to a digital signal input E 4 of the digital/analog converter DAC 1. The controllable switches are driven by n lines connected to a digital signal input E 4 of the digital/analog converter DAC 1. The controllable switches in the current mirror circuit S 3 are driven in accordance with the digital data value Dn that is present and is to be converted, with the result that the summation current output by the mirror circuit S 3 corresponds to the product of the data value Dn present and the reference current Iref output by the reference current source 2. In a current/voltage converter 5 connected downstream, the summation current output by the mirror circuit S 3 is converted into a corresponding voltage and output to a voltage amplifier V 6 connected downstream for setting the output level of the analog output signal. The amplifier V 6 amplifies the voltage UA output by the current/voltage converter 5 and outputs an output voltage K xc2x7UA amplified by a gain factor K to a signal output A 7 of the digital/analog converter DAC 1. At the amplifier V 6, the desired gain factor K can be set via setting lines.
The conventional digital/analog converter DAC 1 illustrated in FIG. 1 has the disadvantage, however, that the programmable amplifier V 6 contained therein has a high current consumption and has a relatively high area requirement when the digital/analog converter 1 is integrated on a chip. Furthermore, the programmable amplifier V 6 causes additional signal distortions of the analog output signal.
The object of the present invention, therefore, is to provide a digital/analog converter which does not require an additional amplifier V for level setting of the analog output signal.
This object is achieved according to the invention by means of a digital/analog converter having the features specified in patent claim 1.
The invention provides a digital/analog converter with programmable gain for converting a digital input signal into an analog output signal, the digital/analog converter having a digital signal input for applying the digital input signal, a reference current source for generating a reference current, the current intensity of the reference current output by the reference current source being adjustable via setting lines, current mirror circuits which mirror the generated adjustable reference current in each case with an associated mirror ratio to form mirror currents, a controllable switching device, which switches through the mirror currents output by the current mirror circuits, in a manner dependent on the digital input signal present at the signal input, to a summation current node for generating a summation current and a current/voltage converter for converting the generated summation current into a corresponding voltage, which is output as an analog output signal via an analog signal output of the digital/analog converter.
The reference current source preferably has a switchable resistor network.
The resistor network preferably contains a plurality of resistors which can be switched via associated switches for altering the reference current flowing through the resistor network.
In this case, the switches of the resistor network can preferably be driven via setting lines.
The current/voltage converter is preferably a resistor.
The current mirror circuits preferably each comprise two current mirror transistors whose control terminals are connected to one another.
In this case, the current mirror transistors are preferably MOSFET transistors.
In an alternative embodiment of the digital/analog converter according to the invention, the current mirror transistors are bipolar transistors.